An electroluminescent (EL) device is a self-light-emitting device which has advantages over other types of display devices in that it provides a wider viewing angle, a greater contrast ratio, and has a faster response time. An organic EL device was first developed by Eastman Kodak, by using small molecules (aromatic diamines) and aluminum complexes in a light-emitting layer [Appl. Phys. Lett. 51, 913, 1987].
The most important factor to determine luminous efficiency in an organic EL device is a light-emitting materials. Until now, fluorescent materials have been widely used as light-emitting material. However, in view of electroluminescent mechanisms, phosphorescent materials theoretically show four (4) times higher luminous efficiency than fluorescent materials. Thus, recently, phosphorescent materials have been investigated.
Iridium(III) complexes have been widely known as phosphorescent material, including bis(2-(2′-benzothienyl)-pyridinato-N,C3′)iridium(acetylacetonate) ((acac)Ir(btp)2), tris(2-phenylpyridine)iridium (Ir(ppy)3) and bis(4,6-difluorophenylpyridinato-N,C2)picolinate iridium (Firpic) as red, green and blue materials, respectively.
In order to improve color purity, luminous efficiency and stability, light-emitting materials can be used as one prepared by mixing a dopant with a host material. In the host material/dopant system, the host material has a great influence on the efficiency and performance of an EL device, and thus is important.
At present, 4,4′-N,N′-dicarbazol-biphenyl (CBP) is the most widely known host material for phosphorescent materials. Further, Pioneer (Japan) developed a high performance organic EL device employing, as a host material, bathocuproine (BCP) or aluminum(III)bis(2-methyl-8-quinolinate)(4-phenylphenolate) (BAlq) which had been a material used for a hole blocking layer.
Though these phosphorous host materials provide good light-emitting characteristics, they have the following disadvantages: (1) Due to their low glass transition temperature and poor thermal stability, their degradation may occur during a high-temperature deposition process in a vacuum. (2) The power efficiency of an organic EL device is given by [(π/voltage)×current efficiency], and thus the power efficiency is inversely proportional to the voltage. Though an organic EL device comprising phosphorescent materials provides better current efficiency (cd/A) than one comprising fluorescent materials, a significantly high driving voltage is required to be applied to an organic EL device, thereby resulting in poor power efficiency (lm/W). (3) Further, the operation lifetime of an organic EL device is short and luminous efficiency is still required to be improved.
International Patent Application Publication No. WO 2006/049013 discloses compounds for organic electroluminescent materials whose backbone has a condensed bicycle group. However, it does not disclose compounds having a nitrogen-containing condensed bicyclic group, which is formed by condensing two 6-membered rings; a carbazolic group; and an aryl or heteroaryl group. Further, an organic EL device comprising said compounds fails to provide good luminous efficiency, operation lifetime and driving voltage.
An object of the present invention is to provide organic electroluminescent compounds imparting excellent luminous efficiency, long operation lifetime and low driving voltage to a device; improved layer combinations using said compounds; and an organic electroluminescent device using said compounds (e.g., in the form of a layer that includes at least one of said compounds as a host material, and also includes a metal complex dopant, specifically an iridium metal complex dopant).